Four-stage high efficiency furnace

ABSTRACT

A four-stage furnace includes a gas flow control assembly operable to provide gas to the furnace at one of four firing rates. The furnace also includes a heat exchanger and an inducer blower for establishing a flow of combustion gas that generates a differential pressure across the heat exchanger. A furnace controller controls the rate at which the gas flow control assembly provides gas to the furnace and controls the inducer blower to establish one of four combustion gas flows that corresponds to one of the four firing rates.

BACKGROUND OF THE INVENTION

The present invention relates to the field of gas furnaces, and in particular to an induced-draft, gas-fired furnace that is adapted to operate at any of four different firing rates.

With a furnace for heating a residential or commercial space, a thermostat senses when the temperature of an interior comfort space is below a set temperature. When the temperature drops below the set temperature, the thermostat provides a call for heat that turns on a gas burner and, after a delay time, a circulation air blower. The gas burner injects flame and heated gas into a heat exchanger, which heats the circulation air that is then returned to the interior space. An induced combustion fan draws combustion gases through the heat exchanger and exhausts them into a vent pipe for discharge to an outside environment. Heating continues until the thermostat senses that the interior room air has been heated above the set point, at which time it opens and ends the call for heat.

Multi-stage furnaces have gas burners that operate at different flow rates, ranging from a high flow rate (i.e., high fire) to varying levels of partial flow rates. The high fire mode is employed when there is a high demand for heating, such as when the partial flow rates fail to increase the interior room air temperature above the set point in an allotted time or when specifically commanded by the thermostat. The partial flow rates are employed when there is a lower demand for heat, and the gas burners provide a corresponding level of fire proportionate to the demand for heat.

The gas burners can be actuated into the various flow rate modes based on the states of combustion pressure switches in the furnace. Combustion pressure switches, which sense the negative pressure in the furnace combustion chamber, serve to turn the burners on only if the inducer fan is bringing enough combustion air in to support the level of fire provided by the burners. In conventional furnace systems, the furnace control is designed to have the same number of pressure switch inputs as the number of operating modes supported. Thus, a change in the number of operating modes in the furnace typically requires a change to the control circuitry of the furnace.

BRIEF SUMMARY OF THE INVENTION

The subject invention is directed to a four-stage furnace including a gas flow control assembly operable to provide gas to the furnace at one of four firing rates. The furnace also includes a heat exchanger and an inducer blower for establishing a flow of combustion air that generates a differential pressure across the heat exchanger. A furnace controller controls the rate at which the gas flow control assembly provides gas to the furnace and controls the inducer blower to establish one of four combustion air flows that corresponds to one of the four firing rates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, cutaway view of a two-stage furnace.

FIG. 2 is a schematic diagram of the gas flow control portion of a furnace for four-stage operation.

FIG. 3 is a graph of the heat exchanger differential pressure and the circulating airflow of a four-stage furnace versus time during high, medium-high, medium-low, and low stages of operation.

DETAILED DESCRIPTION

A four-stage furnace constructed in accordance with the present invention comprises adaptations of a similar conventional two-stage furnace. Accordingly, the following description will first discuss the structure and operation of a two-stage furnace that is known in the art, and then discuss how the structure and operation of a four-stage furnace that is constructed in accordance with the present invention differs from the conventional two-stage furnace.

FIG. 1 is a perspective cutaway view of a conventional two-stage condensing furnace 10. Furnace 10 includes burner assembly 12, burner box 14, air supply duct 16, gas valve 18, primary heat exchanger 20, condensing heat exchanger 24, condensate collector box 26, exhaust vent 28, induced draft blower 30, inducer motor 32, thermostat 34, low pressure switch 42, high pressure switch 44, pressure tubes 46 and 48, blower 50, blower motor 52, and furnace control 54.

Burner assembly 12 is located within burner box 14 and is supplied with air via air supply duct 16. The gases produced by combustion within burner box 14 flow through a heat exchanger assembly, which includes primary or non-condensing heat exchanger 20, secondary or condensing heat exchanger 24, and condensate collector box 26. The gases are then vented to the atmosphere through exhaust vent 28. The flow of these gases, herein called combustion gases, is maintained by induced draft blower 30, which is driven by inducer motor 32. Inducer motor 32 is driven in response to speed control signals that are generated by furnace control 54, in response to the states of low pressure switch 42 and high pressure switch 44, and in response to call-for-heat signals received from thermostat 34 in the space to be heated.

Fuel gas is supplied to burner assembly 12 through gas valve 18, and is ignited by an igniter assembly (not shown). Gas valve 18 may comprise a conventional, solenoid operated two-stage gas valve which has a closed state, a high open state associated with the operation of furnace 10 at its high firing rate, and a low open state associated with the operation of furnace 10 at its low firing rate.

Air from the space to be heated is drawn into furnace 10 by blower 50, which is driven by blower motor 52 in response to speed control signals that are generated by furnace control 54. The discharge air from the blower 50, herein called circulating air, passes over condensing heat exchanger 24 and primary heat exchanger 20 in a counterflow relationship to the flow of combustion gases, before being directed to the space to be heated through a duct system (not shown). While the present invention is described with regard to condensing furnaces (i.e., furnaces that use heat exchanger assemblies that include primary and secondary heat exchangers), it will be appreciated that the concepts of the present invention are also applicable to non-condensing furnaces (i.e., furnaces that have heat exchanger assemblies with only a single heat exchanger unit).

In two-stage furnace 10, inducer motor 32 and blower motor 52 operate at a low speed when the furnace is operating at its low firing rate (low stage operation) and at a high speed when the furnace is operating at its high firing rate (high stage operation). Motors 32 and 52 may be motors that are designed to operate at a continuously variable speed, and to operate at their low and high speeds in response to speed control signals generated by furnace control 54. Furnace control 54 may control the steady state low and high operating speeds of motors 32 and 52 and the times and the rates or torques at which they accelerate to and decelerate from these operating speeds.

The combustion efficiency of an induced-draft gas-fired furnace is optimized by maintaining the proper ratio of the gas input rate and the combustion air flow rate. Generally, the ideal ratio is offset somewhat for safety purposes by providing for slightly more combustion air (i.e., excess air) than that required for optimum combustion efficiency. In furnace 10, the excess air level is kept within acceptable limits in part by low and high pressure switches 42 and 44, respectively, which cause inducer motor 32 to run at speeds that are related to the differential pressure across the heat exchanger assembly. Low and high pressure switches 42 and 44 are connected to burner box 14 through pressure tube 46 to sense the pressure at the inlet of primary heat exchanger 20, and are connected to collector box 26 through a pressure tube 48 to sense a pressure at the outlet of secondary heat exchanger 24.

When thermostat 34 provides a call-for-heat signal to furnace control 54 and furnace control 54 determines that furnace 10 is to operate at its low firing rate, furnace control 54 accelerates inducer motor 32 until it attains a pre-ignition steady state speed corresponding to a heat exchanger differential pressure that is sufficient to actuate low pressure switch 42, but not high pressure switch 44. When this differential pressure has been sustained for a preset time, gas valve 18 assumes its low open state. Under this condition, gas valve 18 supplies gas at the low firing rate to burner assembly 12, which ignites the gas and begins heating the combustion gases passing through the heat exchange assembly. This heating initiates a change in the density of the combustion gases which, in turn, causes an increase in the differential pressure across the heat exchange assembly. The speed of inducer motor 32 is then reduced until it attains a steady state speed value that corresponds to a heat exchanger differential pressure that is somewhat lower than its pre-ignition value. After reducing the speed of inducer motor 32, furnace control 54 provides a signal that causes blower motor 52 to accelerate until it reaches a steady state speed that corresponds to a circulating airflow at which furnace 10 is designed to operate at low stage.

Similarly, when thermostat 34 provides a call-for-heat signal to furnace control 54 and furnace control 54 determines that furnace 10 is to operate at its high firing rate, furnace control 54 accelerates inducer motor 32 until it attains a pre-ignition steady state speed that corresponds to a heat exchanger differential pressure that is sufficient to actuate both low pressure switch 42 and high pressure switch 44. When this differential pressure has been sustained for a preset time, gas valve 18 assumes its high open state. Under this condition, gas valve 18 supplies gas at the high firing rate to burner assembly 12, which ignites the gas and begins heating the combustion gases passing through the heat exchanger assembly. This heating initiates a change in the density of the combustion gases which, in turn, causes an increase in the differential pressure across the heat exchange assembly. The speed of inducer motor 32 is then increased to attain a steady state speed value that corresponds to a heat exchanger differential pressure that is somewhat higher than its pre-ignition value. After increasing the speed of inducer motor 32, furnace control 54 causes blower motor 52 to accelerate to a steady state speed value that corresponds to the circulating airflow value at which furnace 10 is designed to operate.

In order to reduce the operating cost of furnace 10 by improving its annual fuel utilization efficiency (AFUE), the combustion gas flow for furnace 10 may be adapted to provide for intermediate stages of operation between the low and high stages of operation. In accordance with the present invention, furnace 10 is modified to provide four-stages of operation including low, medium-low, medium-high, and high states of operation. As will be described in more detail below, this may be accomplished by adapting pressure switches 42 and 44 for use in association with two stages of operation each, adding a throttling valve to the gas control portion of furnace 10 to provide an additional gas regulating component, and programming furnace control 54 to provide control for four stages of operation.

FIG. 2 is a schematic view of gas flow portion 60 of a furnace that is configured for four-stage operation. Similar components between gas flow portion 60 and the gas flow portion of furnace 10 (FIG. 1) are labeled with like numbers, including gas valve 18, inducer motor 32, thermostat 34, low pressure switch 42, high pressure switch 44, blower motor 52, and furnace control 54. Gas flow portion 60 also includes throttling valve relay 62 (including switch 62 a and solenoid 62 b), gas valve relay 66 (including switch 66 a and solenoid 66 b), and throttling valve 68.

The operation of gas control portion 60 is monitored and controlled by furnace control 54, which includes control CPU 58 which has connection pins, labeled P1, P2, P3, P4, P5, P6, P7, P8, P9, and P10, to provide signals to and receive signals from the components of gas flow portion 60. Thermostat 34 is connected to pin P1 to communicate with control CPU 58, and power is supplied from a 24-VAC transformer secondary to thermostat 34 and to pin P2 of control CPU 58. Relay solenoids 62 b and 66 b are connected to pins P4 and P8, respectively, to receive energizing signals from control CPU 58. The pole of single pole, single throw relay switch 62 a is connected to pin P3, and the normally closed contact side of relay switch 62 a is connected to throttling valve 68. The poles of single pole, single throw pressure switches 42 and 44 are also connected to pin P3 of control CPU 58. The output contact of low pressure switch 42 is connected to pin P7 to provide an ON/OFF pressure signal to control CPU 58. The pole of relay switch 66 a is also connected to pin P7, and the normally open output contact of relay switch 66 a is connected to pin P6 and to main and redundant solenoids 70 and 72 of gas valve 18. The output of high pressure switch 44 is connected to pin P5 and to high-fire solenoid 74 of gas valve 18. Throttling valve 68 is connected between the output contact of normally closed relay switch 62 a and ground. Control CPU 58 provides control signals to inducer motor 32 and blower motor 52 via pins P9 and P10, respectively. It should be noted that the schematic in FIG. 2 only shows the connectivity of components of gas flow portion 60 in the furnace, and components from other portions of a heating, ventilation, and air conditioning (HVAC) system may also be connected to and controlled by furnace control 54. However, these components are omitted from FIG. 2 for clarity.

In order to provide four stages of operation, pressure switches 42 and 44 are each associated with two stages of operation. More specifically, pressure switch 42 is used in establishing and maintaining low and medium-low stages of operation of the furnace, while pressure switch 44 is used in establishing and maintaining medium-high and high stages of operation. To accomplish this, pressure switch 42 is configured to actuate when inducer motor 32 reaches a speed that establishes a differential pressure across the heat exchanger corresponding to low and medium-low stage operation. Pressure switch 44 is configured to actuate when inducer motor 32 reaches a speed that establishes a differential pressure across the heat exchanger corresponding to medium-high and high stage operation, but not at differential pressures corresponding to low and medium-low stage operation. When actuated, pressure switches 42 and 44 provide signals via pins P7 and P5, respectively, to control CPU 58.

Throttling valve 68 may comprise a multi-stage throttling valve having at least a first, high open state that provides a low resistance to the flow of gas, and a second, low open state that provides a relatively high resistance to the flow of gas. Throttling valve 68 is disposed in fluidic series between burner box 14 and gas valve 18 (FIG. 1). When the solenoid of throttling valve 68 is de-energized, it is in its low open state, and when the solenoid of throttling valve 68 is energized, it is in its high open state. The open state of throttling valve 68 is a function of the state of throttling valve relay 62, which is controlled by control CPU 58 using a time based staging algorithm that determines staging based on the duration of the call-for-heat signals provided by thermostat 34. As will be described in more detail below, control CPU 58 controls the open states of gas valve 18 and throttling valve 68 to provide four firing rates corresponding to four stages of operation.

Control CPU 58 is programmed to control inducer motor 32 and blower motor 52, in conjunction with gas valve 18 and throttling valve 68, to provide four stages of operation in the furnace. FIG. 3 is a graph of the heat exchanger differential pressure (HXDP) and the circulating airflow, expressed in cubic feet per minute (Blower CFM), in a four-stage furnace as controlled by control CPU 58. The left-side vertical axis of FIG. 3 may also be regarded as showing the speed of inducer motor 32, since the magnitude of the HXDP is related to the speed of inducer motor 32. The HXDP and Blower CFM are plotted versus the time elapsed since the call-for-heat signal was received from thermostat 34. The operation of a furnace including gas control portion 60 will now be described for each of the four stages of operation with reference to FIG. 3.

When thermostat 34 provides a call-for-heat signal to furnace control 54 and control CPU 58 determines that the furnace should operate at its low or medium-low stage of operation, control CPU 58 then accelerates inducer motor 32 until it attains a pre-ignition steady state speed corresponding to a heat exchanger differential pressure, HXDP-ML/L-1, that is sufficient to actuate low pressure switch 42, but not high pressure switch 44. This provides power at the pole of relay switch 66 a.

When differential pressure HXDP-MUL-1 has been sustained for a preset time, gas valve 18 and throttling valve 68 assume states that correspond to the medium-low firing rate for ignition. The medium-low firing rate is used for ignition of both the low and medium-low firing rates because ignition at the low firing rate may not be possible for ignition (but is sufficient to support combustion after ignition). To provide the medium-low firing rate, control CPU 58 energizes solenoid coil 66 b to close relay switch 66 a. When relay switch 66 a is closed, power is provided to main and redundant solenoids 70 and 72, which causes gas valve 18 to assume its low open state. In addition, control CPU 58 keeps relay solenoid 62 b de-energized, which maintains switch 62 a in its normally closed state and energizes the solenoid of throttling valve 68. The combination of gas valve 18 in its low open state and throttling valve 68 in its high open state provides the medium-low firing rate. In one embodiment, gas is supplied at the medium-low firing rate at 60% of the high firing rate.

Gas valve 18 and throttling valve 68 supply gas at the medium-low firing rate to burner assembly 12, which ignites the gas and begins heating the combustion gases passing through the heat exchange assembly. This heating initiates a change in the density of the combustion gases that, in turn, causes an increase in the differential pressure across the heat exchanger assembly, as shown in FIG. 3. At this time, for a medium-low call for heat, control CPU 58 maintains gas valve 18 and throttling valve 68 to continue to provide gas at the medium-low firing rate. For a low call for heat, control CPU 58 energizes relay solenoid 62 b to open relay switch 62 a and de-energize the solenoid of throttling valve 68. The causes throttling valve 68 to assume its low open state which, in combination with the low open state of gas valve 18, provides the low firing rate. In one embodiment, gas is supplied at the low firing rate at 45% of the high firing rate.

For both medium-low and low firing rates, the speed of inducer motor 32 is then reduced until it attains a steady state speed value that corresponds to a heat exchanger differential pressure, HXDP-ML/L-2 that is somewhat lower than its pre-ignition value. For the medium-low firing rate, this heat exchanger differential pressure is maintained until operation of the furnace is terminated or until control CPU 58 determines that it needs to operate at another stage. For the low firing rate, the speed of inducer motor 32 is again reduced to its low stage steady state speed to provide a heat exchanger differential pressure, HXDP-L-3, corresponding to low stage operation of the furnace. The heat exchanger differential pressure HXDP-L-3 for low stage operation is still sufficient to maintain the closed state of pressure switch 42.

Control CPU 58 then provides a signal that causes blower motor 52 to accelerate until it reaches a steady state speed to provide a circulating airflow corresponding to the stage of operation. In particular, for low stage operation, blower motor 52 is accelerated to provide a circulating airflow BCFM-L at steady state speed, and for medium-low stage operation, blower motor 52 is accelerated to provide a circulating airflow BCFM-ML at steady state speed.

When thermostat 34 provides a call-for-heat signal to control CPU 58 and control CPU 58 determines that the furnace is to operate at its medium-high or high stage of operation, control CPU 58 then accelerates inducer motor 32 until it attains a pre-ignition steady state speed corresponding to a heat exchanger differential pressure, HXDP-H/MH-1, that is sufficient to actuate both low pressure switch 42 and high pressure switch 44. This provides power at the pole of relay switch 66 a and energizes high-fire solenoid 74.

When differential pressure HXDP-H/MH-1 has been sustained for a preset time, gas valve 18 and throttling valve 36 assume states that correspond to the high firing rate for ignition. To provide the high firing rate, furnace control 54 energizes solenoid coil 66 b to close relay switch 66 a. When relay switch 66 a is closed, power is provided to main and redundant solenoids 70 and 72, which, in conjunction with the energized state of high-fire solenoid 74, causes gas valve 18 to assume its high open state. In addition, control CPU 54 keeps relay solenoid 62 b de-energized, which maintains switch 62 a in its closed state and energizes the solenoid of throttling valve 68. The combination of gas valve 18 in its high open state and throttling valve 68 in its high open state provides the high firing rate.

Gas valve 18 and throttling valve 68 supply gas at the high firing rate to burner assembly 12, which ignites the gas and begins heating the combustion air passing through the heat exchanger assembly. This heating initiates a change in the density of the combustion air that, in turn, causes an increase in the differential pressure across the heat exchanger assembly, as shown in FIG. 3. At this time, for a high call for heat, control CPU 58 maintains gas valve 18 and throttling valve 68 to continue to provide gas at the high firing rate, and inducer motor 32 is accelerated until it attains a steady state speed corresponding to a heat exchanger differential pressure of HXDP-H-2.

For a medium-high call for heat, furnace control 54 energizes relay solenoid 62 b to open relay switch 62 a and de-energize the solenoid of throttling valve 68. The causes throttling valve 68 to assume its low open state which, in combination with the high open state of gas valve 18, provides the medium-high firing rate. In one embodiment, gas is supplied at the medium-high firing rate at 75% of the high firing rate. Inducer motor 32 is then decelerated until it to attains a steady state speed corresponding to a heat exchanger differential pressure of HXDP-MH-2.

Control CPU 58 then provides a signal that causes blower motor 52 to accelerate until it reaches a steady state speed to provide a circulating airflow corresponding to the stage of operation. In particular, for medium-high stage operation, blower motor 52 is accelerated to provide a circulating airflow BCFM-MH at steady state speed, and for high stage operation, blower motor 52 is accelerated to provide a circulating airflow BCFM-H at steady state speed.

A two-stage furnace modified as described above to provide four stages of operation offers several advantages. For example, by allowing the gas control portion of a two-stage furnace to be adapted to provide for intermediate stages of operation, the operating cost of the furnace is reduced without requiring a hardware redesign of the furnace control circuit board or the entire furnace unit. In addition, since the four-stage furnace will ordinarily spend most of its time operating at low stage, it will operate at a lower average noise level and provide a higher average thermal comfort level. Furthermore, since the amount of electrical power used during low stage operation is lower than that of other stages of operation, the furnace will operate at a reduced electrical operating cost.

In summary, the subject invention is directed to a four-stage furnace including a gas flow control assembly operable to provide gas to the furnace at one of four firing rates. The furnace also includes a heat exchanger and an inducer blower for establishing a flow of combustion air that generates a differential pressure across the heat exchanger. A furnace controller controls the rate at which the gas flow control assembly provides gas to the furnace and controls the inducer blower to establish one of four combustion air flows that corresponds to one of the four firing rates. By allowing the gas control portion of a two-stage furnace to be adapted to provide for four stages of operation, the operating cost of the furnace is reduced without requiring a hardware redesign of the furnace control circuit board or the entire furnace unit.

Although the present invention has been described with reference to examples and preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. 

1. A four-stage furnace comprising: a gas flow control assembly operable to provide gas to the furnace at one of four firing rates; a heat exchanger; an inducer blower for establishing a flow of combustion gases that generates a differential pressure across the heat exchanger; and a furnace controller for controlling the rate at which the gas flow control assembly provides gas to the furnace and for controlling the inducer blower to establish one of four combustion gas flows that corresponds to one of the four firing rates.
 2. The furnace of claim 1, wherein the gas flow control assembly comprises: a main gas valve having first and second open states; and a throttling valve connected in fluidic series with the main gas valve having at least first and second open states.
 3. The furnace of claim 1, and further comprising: a pressure switch assembly including a first pressure switch and a second pressure switch, wherein the pressure switch assembly senses differential pressure across the heat exchanger and generates first and second pressure signals that vary in accordance with the differential pressure sensed across the heat exchanger.
 4. The furnace of claim 3, wherein the furnace controller controls the furnace to operate at first and second firing rates as a function of the first pressure signal from the first pressure switch, and wherein the furnace controller controls furnace to operate at third and fourth firing rates as a function of the second pressure signal from the second pressure switch.
 5. The furnace of claim 1, wherein the combustion gas flows established by the inducer blower for ignition at first and second firing rates are substantially equal, and wherein the combustion gas flows established for ignition at third and fourth firing rates are substantially equal.
 6. The furnace of claim 1, wherein the combustion gas flows established during steady state operation of the furnace at the four firing rates have different respective magnitudes.
 7. The furnace of claim 1, and further comprising: a circulating air blower that is controlled by the furnace controller for establishing a flow of circulating air from the furnace.
 8. The furnace of claim 7, wherein the circulation air flows established during steady state operation of the furnace at the four firing rates have different respective magnitudes.
 9. A four-stage furnace including an inducer blower for establishing a flow of combustion gas, a circulating air blower for establishing a flow of circulating air, and a heat exchanger, the furnace comprising: a gas flow control assembly for controlling a rate at which gas is supplied to the furnace to one of a first firing rate, a second firing rate, a third firing rate, and a fourth firing rate; a pressure switch assembly including a first pressure switch and a second pressure switch, wherein the pressure switch assembly senses differential pressure across the heat exchanger and generates pressure signals that vary in accordance with the differential pressure sensed across the heat exchanger; and a furnace controller for controlling the rate at which the gas flow control assembly supplies gas to the furnace, for controlling speeds of the inducer blower and the circulating air blower, and for receiving the differential pressure signals from the pressure switch assembly.
 10. The furnace of claim 9, wherein the gas flow control assembly comprises: a main gas valve having first and second open states; and a throttling valve connected in fluidic series with the main gas valve having at least first and second open states.
 11. The furnace of claim 9, wherein the furnace controller controls the furnace to operate at the first and second firing rates as a function of the first pressure signal from the first pressure switch, and wherein the furnace controller controls furnace to operate at the third and fourth firing rates as a function of the second pressure signal from the second pressure switch.
 12. The furnace of claim 9, wherein a magnitude of the combustion gas flow established for ignition at the first and second firing rates are substantially equal, and wherein a magnitude of the combustion gas flow established for ignition at the third and fourth firing rates are substantially equal.
 13. The furnace of claim 9, wherein the combustion gas flows established during steady state operation of the furnace at the first, second, third, and fourth firing rates have different respective magnitudes.
 14. The furnace of claim 9, wherein the circulation air flows established during steady state operation of the furnace at the first, second, third, and fourth firing rates have different respective magnitudes.
 15. A furnace comprising: a main gas valve having low open state and a high open state; a throttling valve connected in fluidic series with the main gas valve and having a low open state and a high open state; and a furnace controller for operating the main gas valve and the throttling valve to supply a gas to the furnace at one of a low firing rate, a medium-low firing rate, a medium-high firing rate, and a high firing rate.
 16. The furnace of claim 15, wherein the main gas valve and the throttling valve are in their low open states for the low firing rate, the main gas valve is in its low open state and the throttling valve is in its high open state for the medium-low firing rate, the main gas valve is in its high open state and the throttling valve is in its low open state for the medium-high firing rate, and the main gas valve and the throttling valve are in their high open states for the high firing rate.
 17. The furnace of claim 15, and further comprising: a heat exchanger; and an inducer blower controlled by the furnace controller for establishing a flow of combustion gas that generates a differential pressure across the heat exchanger that corresponds to one of the four firing rates.
 18. The furnace of claim 17, wherein the combustion gas flows established by the inducer blower for ignition at the low and medium-low firing rates are substantially equal, and wherein the combustion gas flows established for ignition at the medium-high and high firing rates are substantially equal.
 19. The furnace of claim 17, wherein the combustion gas flows established during steady state operation of the furnace at the four firing rates have different respective magnitudes.
 20. The furnace of claim 15, and further comprising: a pressure switch assembly for sensing pressure across the heat exchanger and including a low pressure switch and a high pressure switch, wherein the furnace controller controls the furnace to operate at the low and medium-low firing rates as a function of a pressure signal from the low pressure switch, and wherein the furnace controller controls furnace to operate at medium-high and high firing rates as a function of a pressure signal from the high pressure switch. 